NCCER Control Systems And Fundamental Concepts (26211-23) Practice Test

Overload protection in motors can be based on heat or on electromagnetic action. The magnetic overload relay uses the current itself to trigger a trip: when the current exceeds the relay’s rated value, a magnetic field pulls a mechanism to interrupt the circuit. This trip is driven by the electromagnetic force of the current, not by heat, so ambient temperature has little effect on when it trips. In contrast, the other types rely on heat: a bimetallic relay uses two metals that expand at different rates and bend to trip as they heat; a thermal overload relay uses a heater element that simulates motor heat to reach its trip point; a melting alloy relay uses a fusible alloy that melts when it overheats. All of these are influenced by temperature and cooling conditions, which can shift their trip current or time-to-trip. So the overload relay type not affected by temperature is the magnetic overload relay.